PURPOSE: To quicken the rising of a laser by storing a digital value inputted to a D/A converter when the end of leading is discriminated as a digital value inputted to the D/A converter at the succeeding rising of a laser element.

CONSTITUTION: A comparatively large analog value is outputted from a D/A converter 21 by inputting a digital value to the D/A converter 21 at the initial rising of a laser element. Thus, the laser element sends a sufficient optical output for forming an image immediately when the laser element is active. When a difference between a value resulting from digitally converting a monitor voltage being an output of the laser element at an A/D converter 25 and a reference value reaches a preset range, the rising of the laser element is finished and the digital value inputted to the D/A converter 21 is stored in a nonvolatile memory 44 as a digital value inputted to the D/A converter at the succeeding rising of the laser element. Thus, the rising of the laser is quickened.